Tumor angiogenesis, a major requirement for tumor outgrowth and metastasis, is regulated by pro- and antiangiogenic factors. Methylation-associated inactivation of the angiogenesis inhibitor thrombospondin-1 (TSP-1) has been observed recently in some adult tumors. To investigate the role of TSP-1 in pediatric cancer, we examined its pattern of expression and mechanisms of regulation in neuroblastoma (NB). TSP-1 was silenced in a subset of undifferentiated, advanced-stage tumors and NB cell lines. In contrast, most localized tumors expressed this angiogenesis inhibitor, and a significant correlation between morphological evidence of neuroblast differentiation and TSP-1 expression was observed. Luciferase assays demonstrated the presence of nuclear factors required for TSP-1 transcription in both TSP-1-positive and -negative cell lines, but no correlation between TSP-1 promoter activity and the level of TSP-1 mRNA expression was seen. Our studies indicate that the transcriptional silencing of TSP-1 was caused by methylation. TSP-1 promoter methylation was detected in all of the NB cell lines lacking TSP-1 mRNA and in 37% of the NB clinical tumors analyzed. Furthermore, treatment with the demethylating agent, 5-Aza-2'-deoxycytidine (5-Aza-dC), restored TSP-1 expression in NB cell lines. Disrupting methylation with 5-Aza-dC also led to significant inhibition of NB in vivo and re-expression of TSP-1 in a subset of NB xenografts. These results suggest that 5-Aza-dC inhibits NB growth by augmenting the expression of TSP-1 along with other genes that suppress tumor growth. Demethylating agents may prove to be effective candidates for the treatment of children with NB.